In an example of a laser printer, a developing cartridge is installed in a printer body. Toner is included in the developing cartridge. The toner in the developing cartridge is used to form an image on paper. When the toner in the developing cartridge is exhausted, the developing cartridge is taken out of the printer body, and a new developing cartridge is installed in the printer body. If a paper jam occurs in the printer body, the developing cartridge is taken out of the printer body; after the paper jam has been cleared, the developing cartridge may be installed again in the printer body.
A detecting gear is attached to a side surface of the developing cartridge so as to be rotatable about an axis line (rotational axis line) extending in a direction orthogonal to the side surface. The detecting gear has a plate-like detecting gear body and an abutting protrusion formed integrally with the detecting gear body, the abutting protrusion being disposed on an outer side of the detecting gear (on a side of the detecting gear body opposite to the side surface of the developing cartridge). Gear teeth are formed on the circumferential surface of the detecting gear except some portion of the circumferential surface.
With a new developing cartridge, the gear teeth of the detecting gear are engaged with the gear teeth of a transmission gear. When the developing cartridge is installed in the printer body, the driving force of a motor is supplied to the transmission gear, and the driving force is transmitted from the transmission gear to the detection gear through their gear teeth.
Thus, the detection gear rotates, and the abutting protrusion of the detecting gear moves in the rotational direction of the detecting gear due to the rotation of the detecting gear. When the detecting gear further rotates and a missing tooth portion of the detecting gear faces the gear teeth of the transmission gear, the engagement between the gear teeth of the transmission gear and the gear teeth of the detecting gear is released, stopping the rotation of the detecting gear. Accordingly, after the developing cartridge has been installed in the printer body even once and the driving force is supplied to the transmission gear, the engagement between the gear teeth of the transmission gear teeth of the gear teeth of the detecting gear is released and the disengaged state is kept after that.
In the printer body, a sensor that detects the passage of the abutting protrusion is provided, regarding the abutting protrusion as a protrusion to be detected. Whether the developing cartridge is a new one or an old one is determined depending on whether the sensor has detected the passage of the abutting protrusion. Specifically, after the developing cartridge has been installed in the printer body, if the passage of the abutting protrusion is detected by the sensor, the developing cartridge is determined to be new. However, after the developing cartridge has been installed in the printer body, if the passage of the abutting protrusion is not detected by the sensor, the developing cartridge is determined to be old.
If, however, an amount by which the abutting protrusion protrudes from the side surface of the developing cartridge is large, when the developing cartridge is installed in or removed from the printer body, the abutting protrusion may rub against a member in the printer body and may wear out. Further, with the large amount of protrusion of the abutting protrusion is that when the developing cartridge is installed in or removed from the printer body, the abutting protrusion may come into contact with a member in the printer body or may be caught by the member and the abutting protrusion and/or the member in the printer body may thereby be damaged.